The present invention relates to preconditioning of a polishing pad employed in chemical-mechanical polishing. More particularly, the present invention relates to insitu, automatic preconditioning of a polishing pad employed in chemical-mechanical polishing.
Chemical-mechanical polishing (sometimes referred to as "CMP") typically involves mounting a wafer face down on a holder and rotating the wafer face against a polishing pad mounted on a pallet, which in turn is rotating or is in orbital state. A slurry containing a chemical that chemically interacts with the facing wafer layer and an abrasive that physically removes that layer is flowed between the wafer and the polishing pad or on the pad near the wafer. During IC fabrication, this technique is commonly applied to planarize various wafer layers, such as dielectric layers, metallization, etc. During CMP, the particles eroded from the wafer surface along with the abrasives in the slurry tend to glaze or accumulate over the polishing pad, reducing the polishing rate of the wafer surface and producing a non-uniformly polished wafer surface, e.g. the peripheral region of the wafer surface may not be polished to the same extent as the center region of the wafer surface. One way of achieving and maintaining a high and stable polishing rate is by conditioning the polishing pad every time after a wafer has been polished.
FIG. 1 shows part of a conventional chemical-mechanical polishing apparatus 10, which includes wafer cassettes 18, 20, 22, and 24, a robotic arm 16, a polishing pad 12 mounted on a rotating table or pallet 13, a conditioning arm 14, and a conditioning head 26. Cassettes 18, 20, 22, and 24 come equipped with various slots to store wafers of a production lot; such wafers are referred to herein as "production wafers." Slurry is delivered to polishing pad 12 by slurry inlet line 11. A wafer 15 held by a wafer holder 17 is rotatably driven against polishing pad 12 by a motor 19. Wafer holder 17 and motor 19 are positioned with respect to the polishing pad by an arm 21.
Depending on its size, polishing pad 12 may undergo conditioning either after the production wafer is polished or simultaneously while the production wafer is being polished. For convenience, FIG. 1 shows conditioning and polishing occurring simultaneously. Wafer polishing begins when robotic arm 16 takes a production wafer from one of cassettes 18, 20, 22, or 24 and provides that wafer, face down, to wafer holder 17. Motor 19 then rotates wafer 15 (via wafer holder 17) while a different motor rotates polishing pad 12 (via table 13). As wafer polishing proceeds, a slurry is delivered to pad 12 via inlet line 11.
At the appropriate time, conditioning arm 14 is lowered such that conditioning head 26 comes in contact with and engages rotating polishing pad 12. During pad conditioning, conditioning arm 14. pivots on one end, allowing the conditioning head to forcibly sweep back and forth across polishing pad 12 and generate grooves on the polishing pad. Although polishing pad 12 can be provided with grooves or perforations, the effectiveness of such grooves is reduced over time due to normal polishing. The conditioning pad thus serves to reintroduce the grooves or other roughen the pad surface. It accomplishes with ajagged surface such as a wheel having diamond grit. Grooves produced during pad conditioning facilitate the polishing process by creating point contacts between the wafer surface and the pad, increase the pad roughness and allow more slurry to be applied to the substrate per unit area. Accordingly, the grooves generated on a polishing pad during conditioning increase and stabilize the wafer polishing rate.
U.S. Pat. No. 5,216,843 issued to Breivogel et al. describes a structure of one conditioning arm 14 and conditioning head 26. This patent is incorporated herein by reference in its entirety for all purposes.
Typically after polishing the last wafer in the last cassette, polishing pad 12 may sit idle for a period of time, e.g., anywhere from a few seconds to a few hours, before cassettes containing production wafers of a new production lot are queued up for polishing. Idle time may also result from a machine malfunction or routine maintenance. In order to prevent the polishing pad from drying up during the pad idle time, the polishing pad is maintained in a wet soak.
The first few production wafers from the new lot to undergo chemical-mechanical polishing on the polishing pad that has been idle for a period of time, may suffer from "a first-wafer effect." The first-wafer effect refers to a significant difference in polishing results, e.g., material removal rate and uniformity of material removal, obtained for the first wafer compared to that obtained for the subsequent production wafers. It is believed that the significant difference in the polishing results obtained for the first wafer compared to the subsequent production wafers is attributed to the different polishing conditions encountered by the first wafer. Possibly this results from a non-equilibrium situation in which the concentration of the particular material removed from the wafer surface increases during polishing of the first wafer. Once the first few wafers are completely polished, the pad may have a steady concentration of such material. Thus, the polishing conditions stabilize after the first few wafers are polished.
In the wafer fabrication industry, it is common practice to set the chemical-mechanical polishing conditions for the subsequent production wafers based on the results obtained for the first production wafer. Therefore, when the polishing results obtained for the first production wafer vary significantly from that of the subsequent production wafers under the same polishing conditions, the polishing conditions set for the subsequent production wafers may strongly deviate from optimal conditions.
To mitigate the problems of the first wafer effect, blank "preconditioning wafers" may be contacted with a rotating polishing pad. The preconditioning wafers should have a coating of the same or a similar material as that which will undergo polishing on the production wafer surface. After preconditioning with such wafers for a certain length of time, the first production wafer is installed in the wafer holder and polished. Because the preconditioning wafer has "preconditioned" the pad, the first wafer effect is reduced or eliminated. This preconditioning procedure is currently implemented in a somewhat cumbersome manner. For example, a worker in the fabrication facility may first transport a cassette containing preconditioning wafers from a remote location to the polishing apparatus, where the preconditioning wafers then undergo chemical-mechanical polishing to precondition the polishing pad. Further, about 3 or 4 preconditioning wafers may be required before the polishing pad is effectively preconditioned to reduce the first-wafer effect.
As should be apparent, the current pad conditioning process suffers from several draw backs. For example, the pad preconditioning process described above is a time-consuming and arduous task. It requires transporting the preconditioning wafers to the CMP apparatus, occupying valuable space in wafer cassettes with these wafers, and installing these wafers. All this is done while a new lot of production wafers must wait to undergo polishing. Furthermore, the preconditioning wafers must be periodically evaluated, reworked or redeposited with the appropriate coating to maintain effective pad preconditioning. This translates into reduced throughput for the polishing process. The maintenance of the preconditioning wafers can also be an expensive proposition.
What is therefore needed is an improved apparatus and process of preconditioning a polishing pad to avoid the labor intensive steps of the current process and provide a higher throughput at reduced cost.